Ultrasound offers a fast, non-invasive, and cost-effective imaging and treatment modality in modern medical practices. Its applications have been rapidly growing with the advances of phased array fabrication and electronics technologies. Ultrasound waves and ultrasound energy fields are projected from an ultrasound transducer into a volume undergoing imaging or therapy. A transducer operates on the principle of converting input electrical driving signal energy to output ultrasound energy because the material of which the transducer is made undergoes mechanical dimensional changes commensurate with the input driving electrical signal. Also, depending on the application, a transducer can convert incident ultrasound energy into electrical energy that can be measured, through a converse mechanical to electrical transduction process whereby dimensional compression by the incident acoustic waves excites or induces an electrical response in the material of the transducer. Typical materials used to manufacture ultrasound transducer elements are piezoelectric crystal materials such as lead zirconate titanate (PZT) and similar materials.
In transmission mode, the ultrasound energy is emitted from a face of a transmitting transducer and propagates according to the known laws of acoustic energy propagation in the medium of choice, typically a fluid or viscoelastic or other medium permitting propagation of ultrasonic sound waves. The tissue of a patient undergoing imaging or therapy with a transducer device or array is sometimes approximated as a viscoelastic fluid and has acoustic parameters such as sound speed and absorption coefficients that can be determined and affect the way in which the ultrasound waves move through the body of the patient.
A plurality of ultrasound sources or transducer elements may be grouped into arrays, which have been produced in one and two dimensions. By controlling the electrical drive signals to each of (or groups of) the ultrasound elements of the array, the resultant emitted sound fields from the array as a whole can be controlled and directed in space and time. Both the amplitude and the phase of the electrical driving signal applied to elements of an array are controlled, at the individual element level, using a computer controlled driving circuit.
When the size of a transducer element is sufficiently small it acts as a point source of ultrasound when observed from a relative distance away from the transducer. The so-called far-field behavior of an ultrasound array is often approximated by considering the cumulative effect from each member transducer of the array. For arrays of many transducer elements the principle of superposition generally applies, at least as a first approximation in linear systems, whereby the total ultrasound field is derived by additively summing the effect of the individual elements of the array to obtain a net field of the total array at any instance in space and time. Phased arrays therefore allow an ultrasound beam to be created (having a given spatial distribution) and allow for electronically steering and focusing the beam in a target volume without the need for mechanical means to steer or reposition the transducer. One can precisely and rapidly control acoustic power deposition at multiple locations using phase aberration correction algorithms in order to steer and focus the beam through different tissue layers, such as fat and muscles. Taking advantage of these unique capabilities, fast volumetric imaging and coagulation of cancer tissue seated deeper in the body can be readily performed.
The construction of phased arrays that allow flexible and precise beam formation and steering can involve complex and sophisticated design and manufacturing steps. One design criteria that is sometimes used in ultrasound array design is that the center-to-center spacing (or pitch) between the array elements should be equal to or smaller than half the wavelength to avoid unwanted secondary peaks, such as grating lobes. However, with increased frequency (i.e., reduced wavelength) and a change in array configuration from one dimension to two, the phased array will have an increased number of small elements. A consequence of the small element size is not only the increased complexity of electrical connections to the individual elements but also increased electrical impedance of the elements.
The large electrical impedance of the small array elements can result in an electrical impedance mismatch between an RF driving system (source), generally 50 W, and the array elements. In diagnostic phased arrays, this impedance mismatch causes low acoustic power output in the transmit mode, and consequently poor sensitivity and signal-to-noise ratio (SNR) on the receive mode. Similarly, for high power therapeutic arrays, it can result in poor electrical-to-acoustic power conversion. The traditional solution for the problem is to employ an electrical impedance matching circuit for each element. Since this is accompanied by high manufacturing cost, the traditional method is not generally ideal or efficient for a phased array with a large number of elements. For this reason, the elements are usually designed to have electrical impedances close to the source impedance in order to maximize power transmitted to the elements without using matching circuits.
Attempts have been made to reduce the electrical impedance of array elements instead of using electrical matching circuits. Some methods seek to stack multiple layers (N layers) of piezoelectric material using the thick film process of tape casting to decrease the element's total electrical impedance by a factor of N2. However, the manufacturing process for this method is complicated and expensive. Similarly, bonded multilayer ceramics and composites using a dice-and-fill method may sometimes improve the electrical power transmitted to the array elements. Although the complexity of the fabrication process may be improved, there arise other problems with alignment and delamination of the bonding layers.
An improved transducer design and method of making such transducers and arrays of the same are needed and useful in at least the fields of ultrasonics, medical imaging, ultrasound therapy, and other medical and industrial applications of acoustic transducer technology.